1. Field of the Invention
At least one aspect of this invention relates to a method for enhancing adhesion of metal particles to ceramic models and, more particularly, a method for enhancing adhesion of sprayed metal to ceramic model spray surfaces using spray forming techniques.
2. Background Art
Spray forming techniques for producing steel prototype tooling, such as dies and molds, are commonly utilized in the automotive industry as well as other industries. One typical spray forming technique includes the following steps: (1) casting a ceramic model containing a spray surface about a mold containing a master pattern of the tool to be produced; (2) spraying metal particles onto the spray surface so that the metal particles adhere; (3) allowing the sprayed metal particles to cool thereby forming a metal deposit having the general shape characteristics of the master pattern; and (4) separating the metal deposit from the ceramic model.
During the spraying step, the metallic particles may not adhere sufficiently to the ceramic model. In some instances, the metal deposit may separate from the spray surface during spraying thereby causing the edges of the metal deposit to curl upward and/or the formation of air pockets between the metal deposit and the spray surface. Consequently, the metal deposit may not resemble the general shape characteristics of the master pattern. In some extreme cases, the metal deposit must be scrapped due to the severity of separation between the metal deposit and the ceramic model.
Freeze-cast ceramics have been used as ceramic models based in part on their superior adhesion characteristics. One typical freeze-casting process includes the following steps: (1) pouring a slurry of silica sol and filler material into the mold containing the master pattern, (2) lowering the temperature of the mold to freeze the slurry and form a freeze-cast ceramic containing the spray surface, (3) extracting the freeze-cast ceramic from the mold, and (4) sintering the freeze-cast ceramic.
The adhesion-promoting characteristic typical of freeze-cast ceramics can be attributed to a topology produced at the surface of the substrate that accepts the metal particles and forms a strong adhesion bond. The topology, otherwise referred to as a morphology, is produced due to the presence of silica sol flakes in the ceramic slurry during the freeze-casting process. Depending on the process conditions, i.e., thermal gradient in the slurry, freezing rate, concentrations of slurry components, etc., the flakes can form in various shapes, including grains and dendrites. The silica sol flake patterns produced by sintering leave behind fissures in the surface of the ceramic models. FIG. 1 provides a schematic illustration-of an exploded side view of the surface region of a ceramic model with fissures. The sprayed metal particles 12 bleed into the fissures 14, causing tight adherence of the metal deposit to the ceramic model.
The adhesion-promoting characteristic of freeze-cast ceramics is highly desirable for other ceramic models, i.e. ceramic models that do not require freezing. Ceramic models that do not require freezing are generally less expensive to produce than freeze-cast ceramics. Imparting the adhesion-promoting characteristic on non freeze-cast ceramic models would allow the substitution of non freeze-cast ceramic models for freeze-cast ceramics with attendant labor and time savings while sustaining the quality of the metal deposit. It is also desirable to provide a method of further intensifying the adhesion promoting characteristics of freeze-cast substrates.
At least one aspect of the present invention is related to methods for enhancing adhesion of metal particles to ceramic models.
One preferred method embodiment includes providing a ceramic model having a spray surface and modifying at least a portion of the spray surface of the ceramic model to enhance the adhesion of sprayed metal to the spray surface.
The providing step can comprise preparing a ceramic mixture, pouring the ceramic mixture into a mold, and heating the ceramic mixture to provide the ceramic model. The ceramic mixture can include a fibrous material. The modifying step can include removing the fibrous material contained on at least the portion of the spray surface by heating to provide a plurality of fissures. The fibrous material can be polymeric fiber or metal fiber. If the fibrous material is a metal fiber, the modifying step can include drying the ceramic mixture to provide a plurality of protrusions on the spray surface. These protrusions enhance adhesion of sprayed metal to the spray surface.
Alternatively, the modifying step can include applying a spray of liquid nitrogen to at least the portion of the spray surface to form ice crystals and removing the ice crystals contained on at least the portion of the spray surface by sintering to provide a plurality of fissures. These fissures enhance the adhesion of sprayed metal to the spray surface.
In accord with another preferred embodiment, a ceramic model for accepting sprayed metal particles is disclosed which comprises a ceramic model having a spray surface, wherein at least a portion of the spray surface of the ceramic model is modified to enhance adhesion of sprayed metal to the spray surface. The spray surface can be modified by applying an acid to the spray surface to roughen the spray surface to increase the adhesion of sprayed metal.
Once the ceramic model spray surface with enhanced adhesion characteristics is formed, metal particles can be sprayed onto the spray surface using spray forming techniques. Examples of suitable spray forming techniques include, but are not limited to, cold spraying, flame powder, flame wire, arc spraying, plasma spraying, high energy plasma spraying, vacuum plasma spraying, detonation, and high velocity oxyfuel.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.